Creamy polenta serves as an excellent canvas for hearty beef chili and garlicky sauteed spinach and mushrooms. If you make the chili in advance, the rest of the dish comes together quickly enough for a weeknight.
In a pot, bring milk and 2 1/2 cups water to a simmer; season lightly with salt. Whisking constantly, add polenta in a thin stream. Reduce heat to low; cook, whisking occasionally, until polenta is tender and creamy, about 5 minutes. Remove from heat; stir in butter. Season with salt and pepper; cover to keep warm.
In a large skillet over medium-high, heat oil and garlic until fragrant, 1 to 2 minutes. Remove and discard garlic; add mushrooms and cook, stirring, until browned, 10 to 12 minutes. Add spinach; season with salt and pepper, and cook until just wilted, 1 to 2 minutes. Add vinegar; cook 30 seconds. Season with salt and pepper.
Reheat chili. Serve with vegetables over polenta.